230 CIRCULATION. 



the neck, which, in a living animal, will immediately arrest 

 its action, has no effect. On the other hand, poisoning by 

 the sulpho-cyanide of potassium destroys the muscular irrita- 

 bility, and leaves the nerves intact. By these experiments, 

 which we have frequently repeated, we can completely sep- 

 arate the nervous from the muscular irritability, and show 

 their entire independence of each other ; and there is every 

 reason to suppose that the heart, like the other muscles, does 

 not derive its contractility from any other system. 



It is evident, however, that the heart is often powerfully 

 influenced through the nerves. Sudden and violent emotions 

 will occasionally arrest its action, and have been known to 

 produce death. Palpitations are to be accounted for in the 

 same way. Some of the modifications which we have already 

 considered, depending on exercise, digestion, etc., are effected 

 through the nerves ; and it is through this system that the 

 heart, and all the important organs of the body, are made to 

 a certain extent mutually dependent. It becomes interesting, 

 and highly important, then, to study their influences, and 

 follow out, as clearly as possible, the action of the nerves 

 which are distributed to the heart. 



The anatomical connections of the heart with the nervous 

 centres are mainly through the sympathetic and the pneu- 

 mogastric nerves. We can study the influence of these nerves 

 to most advantage in two ways : first, by dividing them and 

 watching the effect of depriving the heart of their influence ; 

 and second, by exciting them by means of a feeble current 

 of galvanism. It is well known that in an animal just killed 

 the " nervous force " may be closely imitated by galvanism, 

 which is better than any other means of stimulation, as it 

 does not affect the integrity of the nerves, and the amount 

 of the irritation may be easily regulated. 1 



1 We shall not discuss the effects upon the heart of sudden destruction of the 

 great nervous centres. It has been" shown that the heart becomes arrested when 

 the brain is crushed, as by a blow with a hammer, when the medulla oblongata or 

 the spinal cord is suddenly destroyed ; and even the crushing of a foot, in the frog, 



